A rock contains 0.9375 mg of uranium the day it was found and it was formed 4.2*10^9 years ago, how much uranium was present in the rock the day it was formed? the half life of uranium is 7*10^8 years.

Question

algtrigcalc · Answer

how do i solve it using the formula ln(final/initial) =-kt
k= decay constant

aaronq · Answer

It's more convenient to rearrange the formula for what you're solving for, then plug in your values.

$\sf\huge  [A]_i=\dfrac{[A]_f}{e^{-kt}}$, where $\sf k=\dfrac{ln(2)}{t_{1/2}}$


$\sf\huge   [A]_i=\dfrac{ 0.9375 ~mg}{e^{-(\frac{ln(2)}{7*10^8 years}* 4.2*10^9 years)} }$


$\sf \huge   [A]_i=59.999~mg \approx 60~ mg$